Hard-metal or cermet bodies which are intrinsically formed with outer layers of different compositions from the interior of the body are described in EP 0 635 580 A1, EP 0 687 744 A2 and EP 0 822 265 A2.
EP 0 635 580 describes a sintered hard-metal alloy containing nitrogen which is comprised of a nickel and cobalt-containing binder phase and a hard material phase which is constituted by carbides of at least two kinds of transition metals from Groups IVa, Va and VIa of the Periodic Table. The binder phase to a depth of 3 .mu.m from the surface has a concentration or proportion greater than 1.1 to 4 times than the binder phase proportion in the overall composition of the body. To greater penetration depths of up to 800 .mu.m, the binder phase concentration decreases to a mean value for the overall composition. In the surface zone with a depth of at most 3 .mu.m, the binder phase proportion is at most 10% below the highest value locally for the binder phase in the aforementioned region between 300 .mu.m and 500 .mu.m.
The hard-material phase has a metal composition Ti.sub.x W.sub.y M.sub.c in which M is a transition metal other than titanium or tungsten from Group IVa, Va or VIa of the periodic system. The composition satisfies the relationship x+y+c=1 whereby: 0.5&lt;x.ltoreq.0.5, 0.05&lt;Y.ltoreq.0.5 and 0.01&lt;c.ltoreq.0.5.
The titanium proportion in the surface zone in this system amounts to at least 1.01 times the mean titanium proportion in the overall alloy composition. The tungsten proportion in this surface region is between 0.1 and 0.9 times the mean tungsten proportion of the overall composition. Up to a penetration depth of 800 .mu.m, the tungsten and the titanium proportions progressively vary until they reach the respective mean values. The region along the surface, i.e. the surface boundary zone region is either free from tungsten carbide particles or contains tungsten carbide particles in a low concentration which does not exceed 0.1 volume % of this region.
EP 0 687 744 A2 also describes a nitrogen-containing sintered hard-metal alloy which has a hard phase making up 75% by weight of the composition and at most 95% by weight thereof and which contains titanium, an element from Group VIa of the periodic system and tungsten carbide, the balance being a binder phase of nickel and cobalt.
The allow contains 5% by weight to 60% by weight titanium in the form of TiC and 30% by weight of 70% by weight of a metal in the form of the metal carbide. The atomic proportion of nitrogen with respect to the total of carbon and nitrogen in the hard phase is between 0.2 and 0.5. The hard-metal alloy has a soft surface layer which is comprised of the binder phase and tungsten carbide. Below this outermost layer there is a layer of 3 .mu.m to 30 .mu.m in thickness which is comprised substantially of tungsten carbide with reduced binder metal proportions.
EP 0 822 265 A2 describes still another nitrogen-containing sintered hard-metal alloy which includes a hard-material phase of tungsten carbide which also contains carbides, nitrides or carbonitrides of at least one of the elements of Group IVa, Group V or Group VIa of the periodic system or corresponding carbonitrides in addition to a binder phase which is primarily composed of nickel and cobalt. The described sintered body has a surface region which has three layers including an outermost layer with a tungsten carbide content between 0 to 30 volume %, the balance binder phase, an intermediate layer which consists of 50 volume % to 100 volume % tungsten carbide, balance binder phase, and a third, innermost, layer with a tungsten carbide proportion between 0 to 30 volume %, the balance binder phase. The outermost and innermost layers have thickness between 0.1 .mu.m and 10 .mu.m while the intermediate layer has a thickness between 0.5 .mu.m and 10 .mu.m.
The sintered bodies described above all have been indicated as useful as cutting tools because of improved mechanical characteristics.